Labeled N-bromoacetyl-3,3',5-triiodo-L-thyronine (BrAcT.sub.3) is widely used as an affinity label for the study of thyroid hormone binding to a variety of receptor sites. This affinity label, as well as its congener N-bromoacetyl-L-thyroxine (BrAcT.sub.4), is usually prepared by a two-step reaction: coupling of bromoacetic acid with N-hydroxysuccinimide by means of dicyclohexylcarbodiimide and reaction of the N-hydroxysuccinimide bromoacetate formed with .sup.14 C-labeled or .sup.125 I-labeled 3,3',5-triiodo-L-thyronine (T.sub.3) or L-thyroxine (T.sub.4). For the synthesis of unlabeled BrAcT.sub.3, the second step is carried out with unlabeled T.sub.3. However, no physicochemical data have been published to prove unequivocally the identity of the synthesized products and several labeled or unlabeled bands or peaks are always obtained when the crude reaction product is being analyzed by thin layer chromatography (TLC) or by high pressure liquid chromatography (HPLC). It is not clear which one of these bands or peaks represent BrAcT.sub.3 or BrAcT.sub.4. Accordingly, there exists a need for a simple method for the synthesis of BrAcT.sub.3 and of carrier-free BrAc[.sup.125 I]T.sub.3.